Frame relay is a telecommunication service designed for data transmission for intermittent traffic between local area networks (LANs) and between end-points in a wide area network (WAN). Frame relay puts data into individually addressed variable-size unit called a frame. A frame size can be as small as a few bytes to as large as a few thousand bytes. The variable frame size means that frame transmit time and delay would be different for different frames. Frame relay does not attempt to correct errors. When an error is detected in a frame, it is simply dropped. The end points are responsible for detecting and retransmitting dropped frames. This speeds up overall data transmission. A Frame relay frame can incorporate packets from different protocols such as Ethernet, X.25, and Internet Protocol (IP). These packets are collected into frames and delivered to the destinations. Transmitting the data in multiple frames gives frame relay statistical multiplexing and port sharing characteristics.
Frame relay technology is based on the concept of using virtual circuit (VC) connections. VC connections are two-way data paths between two ports that act as private lines in the network. There are two types of virtual circuits, switched virtual circuits (SVC) and permanent virtual circuits (PVC). For most services, the network provides a PVC, which means that users see a continuous, dedicated connection without having to pay for a full-time leased line, while the service provider figures out the route each frame travels to its destination. PVC ensures that packets always arrive in the right sequence so that they can be reassembled successfully.
While the number of applications running over networks has grown, the WAN bandwidth has not kept pace. As network traffic increases, frame relay users begin to experience bandwidth limitation. Eventually, the bandwidth in the connection runs out and workflow and productivity are seriously impacted. Multilink frame relay (MFR) was developed to address the need for more access bandwidth to support the ever-increasing number of networked applications. MFR is a packet transport protocol based on the adoption of the Frame Relay Forum's End-to-End MFR (FRF.15) and UNI/NNI MFR (FRF.16) Implementation Agreements. It provides physical interface emulation for frame relay devices by bundling multiple independent links to transport data. Bundling multiple independent links provides a logical link that produces greater bandwidth than any of the individual physical links. For example, multiple 56/64K bit/sec links or multiple T1 links can be bundled together and treated as a single logical link. Multiple 56/64K bit/sec circuits can also be used to provide fractional T1 access without provisioning a T1, and multiple T1/E1 circuits can be used to provide fractional T3/E3.
Using MFR, the frame relay frames are fragmented into multiple fragments based on a fragment size and distributed to the multiple physical links in the link bundle. There is a good chance that, except for a last fragment, all of the fragments will have the same size. One possible approach of distributing the fragments among the physical links in the link bundle is a round robin method where a first fragment is distributed to a first link, a second fragment is distributed to a second link, etc. until each link in the link bundle receives a fragment. The distribution repeats at the first link until all the fragments in the frame are distributed. Although the round robin approach is simple and easy to implement, it is not efficient because it does not take into consideration a possibility where the different links in the link bundle have different link speed. When that is the case, the faster link is often underutilized. In addition, a fragment distributed to a faster link will arrive at a destination faster than a fragment distributed to a slower link. As such, there is a more frequent possibility that the fragments will arrive at the destination out of sequence.